The long-term aim of this research is to develop instrumentation and techniques for quantification in vivo of radioisotope localization in small animals in order to accelerate and simplify the design and evaluation of new radiopharmaceuticals. Animal SPRINT (ASP) is a high-resolution single-photon ring tomograph modeled after SPRINT but specifically designed for imaging 99mTc, 123I, and 131I in mice and rats with millimeter resolution. It is also capable of imaging larger animals such as primates and dogs at reduced resolution. Support is requested in this proposal to complete the final assembly of ASP, to characterize instrument performance, carry out verification tests with phantoms, and to assimilate the tomograph into routine use in our radiopharmaceutical biodistribution laboratory. This instrument will also be used in a separate research project to validate image reconstruction and estimation algorithms now under development using phantoms and small animals.